Pressure vessels are used in a wide variety of applications to store fluids under pressure substantially higher than ambient pressure. Many applications require both lightweight and strong pressure vessels. A common shape for pressure vessel is a cylinder with port openings on both ends. At least one end of the pressure vessel contains an orifice to allow fill and drain of the fluid.
One particular application of pressure vessels is a bladder-type hydraulic accumulators. Accumulators are widely used in industries to dampen pulsations, compensate for thermal expansion, or provide auxiliary power. An accumulator consists of a high pressure vessel in which a substantially non-compressible hydraulic fluid is held under pressure by gas (e.g., nitrogen) that is encapsulated in a rubber bladder. Hydraulic fluid flows into the accumulator and compresses the gas contained inside the bladder by reducing its storage volume thereby storing energy. If the hydraulic fluid is released, it quickly flows out under the pressure of the expanding gas inside the bladder.
A bladder-type accumulator requires a port opening on (at least) one end of the vessel to allow insertion of the rubber bladder. Conventional accumulators are made out entirely of steel and are typically 8 inch (200 mm) in diameter with port openings in the range of 2.5 inch (63.5 mm) to 5 inch (127 mm).
Composite pressure vessels that are used as hydraulic accumulators are typically adapted from heritage designs. Heritage design vessels are generally meant for gaseous storage and have relatively small port openings, e.g., about 2.5 inch (63.5 mm) or less. This small port size relative to the vessel diameter limits the size and thickness of the bladder that can be used in the accumulator.
When the port opening is large, the pressure exerted by the internal pressure can potentially cause a catastrophic failure of the pressure vessel around the port opening under its operating conditions. In some cases, the vessel with larger port opening can experience premature fatigue failure during its life cycle with the failure emanating from the polar neck region. This is because the pressure experienced in or around the area near the port opening is a function of the pressure and the cross-sectional area of the orifice. Thus, the larger the port opening, greater the pressure exerted by the fluid on the orifice.
Accordingly, there is a need for accumulators or pressure vessels in general that can have a large port opening (e.g., orifice) without increasing the risk of catastrophic failure at or near the port opening due to a large blowout load experienced by the pressure vessel.